**Boyle's Law**is one of the gas laws, and relates the volume and pressure of an ideal gas held at a constant temperature.

The law, expressed in symbols, is:

*PV*=*k*

*V*is the volume of gas,

*P*is its pressure, and

*k*is a constant. The volume is expressed in cubic metres or litres and the pressure in pascals when using SI units.

To maintain the constant during an increase in pressure of a gas, at fixed temperature, requires that the volume decrease. Conversely, reducing the pressure of the gas increases the volume.

The exact value of the constant need not be known to make use of the law in comparison between two volumes of gas at equal temperature:

*P*_{1}*V*_{1}=*P*_{2}*V*_{2}